1. Field of the Invention
The present invention relates to a circuit board for an ink jet head that ejects ink for printing, a method of manufacturing the circuit board, and an ink jet head using the circuit board.
2. Description of the Related Art
An ink jet printing system has an advantage of low running cost because an ink jet head as a printing means can easily be reduced in size, print a high-resolution image at high speed and even form an image on so-called plain paper that is not given any particular treatment. Other advantages include low noise that is achieved by a non-impact printing system employed by the print head and an ability of the print head to easily perform color printing using multiple color inks.
There are a variety of ejection methods available for the ink jet head to realize the ink jet printing system. Among others, ink jet heads using thermal energy to eject ink, such as those disclosed in U.S. Pat. Nos. 4,723,129 and 4,740,796, generally have a construction in which a plurality of heaters to heat ink to generate a bubble in ink and wires for heater electrical connection are formed in one and the same substrate to fabricate an ink jet head circuit board and in which ink ejection nozzles are formed in the circuit board on their associated heaters. This construction allows for easy and high-precision manufacture, through a process similar to a semiconductor fabrication process, of an ink jet head circuit board incorporating a large number of heaters and wires at high density. This helps to realize higher print resolution and faster printing speed, which in turn contributes to a further reduction in size of the ink jet head and a printing apparatus using it.
FIG. 1 and FIG. 2 are a schematic plan view of a heater in a general ink jet head circuit board and a cross-sectional view taken along the line II-II of FIG. 1. As shown in FIG. 2, on a substrate 120 is formed a resistor layer 107 as a lower layer, over which an electrode wire layer 103 is formed as an upper layer. A part of the electrode wire layer 103 is removed to expose the resistor layer 107 to form a heater 102. Electrode wire patterns 205, 207 are wired on the substrate 120 and connected to a drive element circuit and external power supply terminals for supply of electricity from outside. The resistor layer 107 is formed of a material with high electric resistance. Supplying an electric current from outside to the electrode wire layer 103 causes the heater 102, a portion where no electrode wire layer 103 exists, to generate heat energy creating a bubble in ink. Materials of the electrode wire layer 103 mainly include aluminum or aluminum alloy.
The ink jet head circuit board employs a protective layer deposited on the heater only to ensure a reduced consumption of electricity by reducing applied electrical energy but also to prevent possible mechanical damages caused by cavitations from repeated creation and collapse of bubbles in ink and also prevent a reduced longevity of the circuit board which may be caused by the heater 102 being broken as they are repetitively applied electric pulse energy for heating.
The protective layer, when viewed from a standpoint of heat or energy efficiency, preferably has a high heat conductivity or is formed thin. On the other hand, the protective layer has a function of protecting electrode wires leading to the heaters 102 from ink. In terms of a probability of defects occurring in layers during the circuit board fabrication process, it is advantageous to increase the thickness of the protective layer. Therefore, to make a balanced tradeoff between energy efficiency and reliability, the protective layer is set to an appropriate thickness.
However, the protective layer is subject to mechanical damages from cavitations caused by creation of bubbles in ink and also to chemical damages caused by chemical reactions between ink components and materials making up the protective layer at high temperatures to which the protective layer's surface in contact with the heaters rises immediately after bubbles are formed. Hence, the function to insulate and protect the wires from ink and the function to protect against mechanical and chemical damages are difficult to achieve at the same time. It is therefore a common practice to form the protective layer on the ink jet head circuit board in a two-layer structure, and to form as an upper layer, a highly stable layer capable of withstanding mechanical and chemical damages and, as a lower layer, a protective insulation layer to protect the wires.
More specifically, it is common practice to form as the upper layer a Ta layer with very high mechanical and chemical stability and, as the lower layer, a SiN or SiO layer which is stable and easy to deposit using the existing semiconductor fabrication equipment. In more detail, a SiN layer is deposited on the wires to a thickness of about 0.2-1 μm as the lower protective layer (protective insulation layer) 108 and then, as the upper protective layer (generally called an anticavitation layer because of its capability to resist possible damages from cavitations) 110, a Ta layer is deposited to a thickness of 0.2-0.5 μm. This structure meets the contradictory requirements of an improved electrothermal conversion efficiency and a longer service life of the ink jet head circuit board on one hand and its improved reliability on the other.
For reduced power consumption and improved heat efficiency of the ink jet head, efforts are being made in recent years to increase a resistance of individual resistors. So, even minute variations in heater size will greatly affect resistance variations among the heaters. If resistance variations result in differences in bubble generation phenomenon among the heaters, not only can the required amount of ink for one nozzle not be stably secured but the amount of ink also varies greatly among the different nozzles, leading to a degradation of printed image quality. Under these circumstances, an improved precision in patterning the electrode wires at the heaters is being called for more than ever.
Ink jet printers, as they proliferate, are facing increasing demands for higher printing resolution, higher image quality and faster printing speed. One of solutions to the demands for higher resolution and image quality involves reducing an amount of ink ejected to form a dot (or a diameter of an ink droplet when ink is ejected in the form of droplets). The requirement for reducing the ink ejection volume has conventionally been dealt with by changing the shape of nozzles (reducing orifice areas) and reducing the area of heater (width W×length L in FIG. 1). As the heater become smaller in size, the relative effect of heater size variations becomes more significant. This constitutes one of factors calling for improved precision of electrode wire patterning at the locations of heater.
On the other hand, from the standpoint of reducing the amount of electricity consumed by the circuit board as a whole, it is important to lower a resistance of electrode wires. Normally, the resistance of electrode wires is reduced by increasing the width of the electrode wires formed on a circuit board. However, given a situation where the number of heaters formed in the circuit board is very large and there is a growing trend for reducing the area of individual heaters, it is becoming more and more difficult to secure enough space to allow the electrode wires to be increased in width without increasing the size of the circuit board. On top of that, increasing the width of electrode wires imposes limitations on high-density integration of small-area heaters or nozzles.
It may be conceived to achieve a reduced resistance of electrode wires by increasing their thickness. This method, however, renders the improvement in the patterning precision of the heaters difficult.
This is explained by referring to FIG. 1 through FIG. 3.
First, in the construction shown in FIG. 1 and FIG. 2, in those areas where the heaters 102 are to be formed, an electrode wire layer 103′ is etched away to expose a resistor layer. Here, considering the coverage of the protective insulation layer 108 and the anticavitation layer 110, the electrode wire layer 103′ is wet-etched into a tapered shape. Since the wet etching proceeds isotropically, errors caused by etching, particularly dimensional tolerance in the longitudinal direction of the heater 102, are proportional to the thickness of the electrode wire layer 103′.
FIG. 3 shows a relation between a thickness of aluminum electrode wire layer and a dimensional tolerance in a direction L, with abscissa representing a multiplication factor of a thickness of 0.3 μm (300 nm) and ordinate representing a dimensional tolerance (μm). As can be seen from this diagram, for a thickness with multiplication factor=1, the dimensional tolerance is 0.5 μm; for a thickness with multiplication factor=1.7, the dimensional tolerance is about 1 μm; and for a thickness with multiplication factor=2.9, the dimensional tolerance is about 2 μm. This shows that as the length L is made smaller to match the reducing area of the heater 102, the influence of tolerance variations increases.
As described above, it is extremely difficult to meet both of the two requirements at the same time, one for increasing the resistance of resistors and reducing the area of heaters and one for increasing the thickness of electrode wires. They in turn require a very high precision of patterning.